1. Field of the Invention
The present invention relates to a reversible thermosensitive coloring composition which utilizes a coloring reaction of an electron donating coloring agent and an electron accepting color developer, a reversible thermosensitive recording material which uses the coloring composition and in which a colored image is repeatedly formed and erased by appropriately heating and cooling the reversible thermosensitive recording material, and a reversible thermal recording method using the recording material.
2. Discussion of the Related Art
A variety of thermosensitive recording materials are well known in which a colored image can be formed by a coloring reaction when an electron donating coloring agent is brought into contact with an electron accepting color developer while heat or the like is applied thereto.
The thermosensitive recording materials have the following advantages over other conventional recording materials:
(1) color images can be rapidly recorded by a relatively simple apparatus without using such complicated steps as developing and fixing;
(2) color images can be recorded without producing noise and environmental pollution;
(3) various color images, e.g., red, blue, violet and black, can be easily obtained;
(4) image density and background whiteness are high; and
(5) the manufacturing cost is relatively low.
Because of these advantages, the above-described thermosensitive recording materials can be widely used, not only as a recording material for price labels in stores, but also as recording materials for copiers, printers for computers, facsimiles, automatic vending machines of tickets, video printers and measuring instruments.
However, the coloring reaction of these thermosensitive recording materials is irreversible and accordingly these recording materials cannot be repeatedly used.
Several thermosensitive recording materials which can reversibly form and erase an image have been proposed. For example, they are as follows:
(1) a thermosensitive recording material which includes gallic acid in combination with phloroglucinol serving as color developers (Japanese Laid-Open Patent Publication No. 60-193691);
(2) a thermosensitive recording material which includes phenolphthalein or Thymolphthalein serving as a color developer (Japanese Laid-Open Patent Publication No. 61-237684);
(3) a thermosensitive recording material which includes an uniform solid solution of a coloring agent, a color developer and a carboxylic acid ester (Japanese Laid-Open Patent Publications No. 62-138556, 62-138568 and 62-140881);
(4) a thermosensitive recording material which includes an ascorbic acid derivative serving as a color developer (Japanese Laid-Open Patent Publication No. 63-173684); and
(5) a thermosensitive recording material which includes bis(hydroxyphenyl)acetic acid or a higher aliphatic amine salt of gallic acid serving as a color developer (Japanese Laid-Open Patent Publication No. 2-188294).
Some of the present inventors, jointly with others, have proposed a reversible thermosensitive coloring composition which includes a coloring agent such as a leuco dye and a color developer such as an organic phosphate compound, an aliphatic carboxylic acid compound or a phenolic compound each of which has a long-chain aliphatic hydrocarbon group, and a reversible thermosensitive recording material using the coloring composition (Japanese Laid-Open Patent Publication No. 5-124360). The reversible thermosensitive recording material can stably repeat image formation and image erasure by being appropriately heated and cooled, and the image-recorded state or the image-erased state can be stably maintained at room temperature. In addition, Japanese Laid-Open Patent Publication No. 6-210954 has disclosed a reversible thermosensitive recording material which includes a specified phenolic compound having a long-chain aliphatic hydrocarbon group serving as a color developer.
Thus, reversible thermosensitive recording materials have been proposed; however, these recording materials do not have requisite properties such as quick erasability and good preservability when they are preserved under various environmental conditions or when light is irradiated thereto.
Because of these reasons, a need exists for a reversible thermosensitive recording material which has good image formation/erasure ability and rapid erasability, and particularly has good preservability even when preserved under high temperature conditions.
Accordingly, an object of the present invention is to provide a reversible thermosensitive coloring composition which has good color formation/erasure ability, rapid color erasability, and good preservability even when preserved under high temperature conditions.
Another object of the present invention is to provide a reversible thermosensitive recording material which has good image formation/erasure ability and rapid image erasability, and particularly has good preservability even when preserved under high temperature conditions.
Yet another object of the present invention is to provide a reversible thermosensitive recording material which has good durability even when repeatedly used for a long time.
Still another object of the present invention is to provide a reversible thermal recording method in which images having good image qualities are rapidly formed and erased even when images are repeatedly formed and erased for a long time.
To achieve such objects, the present invention contemplates the provision of a reversible thermosensitive coloring composition including an electron donating coloring agent and an electron accepting color developer and in which an image is reversibly formed and erased by appropriately heating and cooling the composition, wherein the composition includes a phenolic compound having the following formula (1) as the electron accepting color developer. 
wherein X and Y independently represents a divalent group including a hetero atom; R1 represents a divalent hydrocarbon group having from 1 to 20 carbon atoms; R2 represents a divalent hydrocarbon group having from 2 to 20 carbon atoms; and n and m are an integer of from 1 to 3, and from 1 to 20, respectively; and r is 0 or an integer of from 1 to 3.
In another aspect of the present invention, a reversible thermosensitive recording material is provided which includes a recording layer including the thermosensitive coloring composition and which can reversibly record a color image by being appropriately heated at a temperature not lower than a coloring temperature and erase the image by being heated at a temperature lower than the coloring temperature and not lower than an erasing temperature. Preferably, the recording layer further includes a crosslinked resin, and a color formation/erasure controlling agent having a hetero atom and an alkyl group having carbon atoms not less than 6.
In yet another aspect of the present invention, a reversible thermal recording method is provided, including the steps of discoloring by heating the recording layer of the reversible thermosensitive recording material of the present invention at a temperature lower than a coloring temperature and not lower than an erasing temperature to allow the recording layer to achieve a non-colored state and recording by imagewise heating the recording layer at a temperature not lower than the coloring temperature to form a colored image in the recording layer.
These and other objects, features and advantages of the present invention will become apparent upon consideration of the following description of the preferred embodiments of the present invention taken in conjunction with the accompanying drawing.